Diamond has the following properties:
a. high hardness; PA1 b. excellent wear resistance; PA1 c. small compressibility as well as a small coefficient of thermal expansion; PA1 d. extremely high heat conductivity for its function as an insulator; PA1 e. optical transparency against ultraviolet, visible and infrared rays with a high refractive index; PA1 f. excellent chemical resistance; PA1 g. excellent acoustic wave propagation velocity; and PA1 h. capability of providing semiconductor properties upon doping with a specific impurity.
In consideration of such various properties, attempts have been made to use diamond in various fields. Diamond is now regarded as a necessary and indispensable substance in industry.
As is well known in the art, diamond has been synthesized from a vapor phase by a CVD process such as a microwave CVD process, a thermal filament CVD process or the like. Thus, it has been made possible to synthesize diamond in the form of a film, or to coat the surface of another material with diamond, in order to effectuate the excellent properties of diamond. Following development of such techniques, the range of application of diamond is now increasing.
Among various vapor-phase synthesizing methods, a diamond synthesizing method using dc discharge has already been developed. In a synthesizing method using discharge under a relatively high pressure region of at least 100 Torr, it is possible to synthesize diamond at a high speed. On the other hand, a synthesizing method using dc discharge under a low pressure of 1 to 100 Torr is suitable for forming a diamond film at a low temperature over a wide area, as well as forming a diamond film on a substrate having a three-dimensional configuration.
Although the diamond synthesizing method using a dc discharge under a low pressure has the aforementioned advantages, the growth rate of diamond is slower and the as-formed diamond is inferior in quality compared with the microwave CVD process and the thermal filament CVD process.